Aim To investigate the risk factors and prognosis of decreased left ventricular ejection fraction (LVEF) in patients with ST-segment elevation myocardial infarction (STEMI) after percutaneous coronary intervention (PCI) in left anterior descending artery (LAD) myocardial infarction group (LAD MI group) (LAD or left main artery as culprit vessel) and non-LAD myocardial infarction group (non-LAD MI group) (circumflex or right coronary artery as culprit vessel). Methods 1246 STEMI patients who received emergency PCI in intensive care unit of the Second Affiliated Hospital of Harbin Medical University from January 2017 to December 2018 were selected. Baseline data, past history, laboratory results, coronary angiography results, major adverse cardiac and cerebrovascular event (MACCE) and LVEF measured within 3 days after PCI were recorded. According to the results of angiography, the patients were divided into LAD MI group and non-LAD MI group, and the two groups were separately divided into two subgroups:low LVEF (LVEF＜50%) group and normal LVEF (LVEF≥50%) group. The risk factors and short-term clinical prognosis of LAD MI group and non-LAD MI group were analyzed. Results Multivariate Logistic regression analysis showed that age (OR＝1.024), chest pain time (OR＝1.122), troponin peak (OR＝1.001), high-sensitivity C-reactive protein (hs-CRP) (OR＝1.048), prior myocardial infarction (OR＝2.404), proximal lesion (OR＝1.564) and application of intra-aortic balloon pump (OR＝3.638) were risk factors for reduced LVEF after PCI in LAD MI group. However, in non-LAD MI group, admission heart rate (OR＝1.017), troponin peak (OR＝1.004), uric acid (OR＝1.003), hs-CRP (OR＝1.101), number of disease vessels (OR＝1.786), prior myocardial infarction (OR＝3.025) and preoperative TIMI flow grade 0/1 (OR＝3.237) were risk factors for reduced LVEF after PCI. In the LAD MI group, the incidences of heart failure and cardiogenic shock during hospitalization in patients with reduced LVEF were significantly higher than those in patients with normal LVEF. In the non-LAD MI group, the incidences of heart failure, cardiogenic shock, malignant arrhythmia and mortality rate during hospitalization in patients with reduced LVEF were significantly higher than those in patients with normal LVEF. Conclusions The risk factors of low LVEF after PCI in patients with LAD MI are significantly different from those in patients with non-LAD MI. Patients with low LVEF have worse short-term clinical prognosis and higher incidence of MACCE in hospital after PCI.